The delta variant of the COVID-19 virus is more infectious and more virulent than the alpha variant first identified in the United Kingdom, according to new research.
The good news is that vaccines still work against it, though somewhat less well, the studies say.
"Delta is a much more concerning variant globally, even than the other variants," said Bill Hanage, associate professor of epidemiology at Harvard T.H. Chan School of Public Health's Center for Communicable Disease Dynamics.
As vaccine access moves at a snail's pace outside of wealthier countries, "much of the world remains extraordinarily vulnerable," said William Powderly, director of the Institute for Public Health at Washington University in St. Louis.
The delta variant was first identified in India. It is likely responsible for that country's explosive outbreak, which has set grim world records for the most deaths per day of any country.
When it spread to the U.K., it overtook the fast-spreading alpha variant in a matter of weeks. Delta now causes 90% of new infections in the U.K., according to government figures.
"The speed by which it spread is pretty alarming, really," said Aziz Sheikh, professor of primary care at the University of Edinburgh.
The variant also causes more serious cases of COVID-19. In a study published Monday in The Lancet journal, Sheikh and colleagues found that patients infected with delta were 85% more likely to be hospitalized than they would if infected with alpha, "which is really of concern," he said.
Vaccines still work against it, though not quite as well, the study found. Both the Pfizer-BioNTech and the University of Oxford-AstraZeneca vaccines were about 13% less effective against delta than they were against alpha. Two weeks after the second dose, Pfizer was 79% effective, and AstraZeneca was 60%.
While some vaccinated people may still get sick, they are not likely to get severely ill, according to U.K. government data released Monday. The Pfizer shot was 96% effective, and the AstraZeneca vaccine was 92% effective against hospitalization after two doses.
"These are still remarkably effective" vaccines, Sheikh said. And they substantially lower the risk that a vaccinated person will get infected, pass on the virus, end up in the hospital or die. "They protect all round, and that equation remains true with the delta variant, as well."
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